1. Technical Field
The present invention relates to laser rangefinder receiver preamplifiers, and more particularly to an improved preamplifier having high gain, insensitivity to overload, and fast recovery time.
2. Background Art
Laser rangefinders are used to measure distance, and in the prior art, a suitable laser transmitter is used to beam a high intensity pulse of light onto a selected target. The light scattered from the target is detected by an optical receiver, normally co-located with the laser transmitter. By measuring the transit time between the transmitted laser pulse and the received echo, the range to the target can be determined using a time-interval counter.
Silicon avalanche photo-detectors have been commonly used to convert the echoed light pulse into an electrical current. These detectors provide amplification of the signal and therefore the noise contribution of the preamplifier is not critical in determining sensitivity.
Recently, laser rangefinders have been designed to function at wavelengths of 1.5 microns and longer in an effort to substantially reduce the hazard to human eyes caused by viewing of the laser beam. The popular and efficient silicon avalanche detectors do not function at these wavelengths, thus necessitating the use of other types of detectors, such as those made from germanium or indium gallium arsenide. The responsivity of these eyesafe wavelength detectors is much less than the silicon avalanche detectors, and the ability of the rangefinder to function effectively with the weak echoes associated with long range targets requires the use of a low-noise, high-gain preamplifier.
Avalanche detectors require precise control of bias voltage, necessitating complex circuits. The use of low gain detectors, for example silicon PIN type, avoids this complexity, but requires a low-noise, high gain preamplifier.
The laser pulse widths used for rangefinding are normally between 5ns and 30ns wide, requiring a typical receiver bandwidth of 5 MHz to 30 MHz for amplification. The noise of the preamplifier is dominated by the thermal noise of the detector load resistor or the preamplifier transimpedance resistor. It is well known that for a low current noise, this resistance must be high. Such receivers using high resistor values generating low noise are used in fiber optics communication.
In a laser rangefinder receiver, the magnitude of the echo pulse varies over many orders of magnitude due to target reflectivity, target size, weather, laser power, and target range. Especially at short ranges, the high-gain, high-impedance preamplifier that is typically utilized to achieve low noise may overload due to excessive detector current. This causes amplifier saturation and results in blocking of targets during the overload condition. The overload may be caused by backscatter from common transmit and receive optics, mist, retroreflecting targets, or as a result of objects close to, or partially intercepting, the beam. Such blocking results in an inability of the rangefinder to measure the distance to certain targets.
There is therefore an unfilled need for a laser rangefinder receiver that has high gain, insensitivity to overload and fast recovery time in cases where silicon avalanche detectors cannot be used due to wavelength incompatibility or a requirement for simplicity.